Steer control for a track-laying vehicle

ABSTRACT

An electronic control circuit combined with an electro-hydraulic transducer regulates right and left steering clutches in a crossdrive transmission for a track-laying vehicle to effect steering by driving. The control inputs are steer lever position and track speeds so that a closed loop control is provided for controlling track speed differential as a function of actual relative track speeds.

United States Patent 91 Sanders et al.

[4 1 Apr. 17, 1973 STEER CONTROL FOR A TRACK- LAYING VEHICLE Inventors:Robert K. Sanders, Whitestown;

Jerry R. Marlow, Greenwood; Kenneth A. Dornfeld, Indianapolis, all ofMotors Corporation, Detroit, Mich.

May 13, 1971 Appl. No.: 142,880

U.S. Cl. ..180/6.7 Int. Cl. ..B62d 11/08 Field of Search 180/1 R, 6.2,6.5,

References Cited UNITED STATES PATENTS l 1/1966 Shepherd ..180/6.2 X

3,543,654 12/1970 Long et a1 l 80/6.5 X 3,620,319 11/1971 Armasow et al1 80/6.7 X 2,930,257 3/1960 Christenson ..74/720.5

Primary Examiner-Kenneth H. Betts Assistant Examiner-Leslie J. PapernerAttorney-Jean L. Carpenter, Paul Fitzpatrick and Warren D. Hill [5 7]ABSTRACT 5 Claims, 11 Drawing Figures CLUTCH T COMPUTER SUMMING STEER C.CIRCUIT LEVER /d TRANSDUCER INTEGRATOR i [/1 RIGHT TRACK M SPEED SENSORFASTEST SPEED COMPUTER 14 1 :2 vkin My 30 RPM A LEFT TRACK QE S SWITCHSPEED SENSOR COMPUTER j! RIGHT .532: CONTROL 5! r LEFT 5 OUTPUT STEERDRIVER CLUTCH NON-CONTROLLING CONTROL PATENTEDAPRIYIBYS 3.727710 SHEET 1UF 3 76 GATE CONTROL &

TRiGGER 7i r v #4 J 7 H- o v L #5 STEER SUMMING LEVER r'\ TRANSDUCER- Iv :4 A

' |NTEGRATOR 1a A: Us L a; RIGHT TRACK 3%& Z /8 SPEED :aENsOR FASTECONVERTER ST COMPUTER FREQUENCY 1 LEFT TRACK TO VOLTAGE Ag??? sPEEPSENSOR CONVERTER COMPUTER a /z' V; T

F x I. Q 7 I GATE f0 1 6' RIGHT sTEER OUTPUT I GATE CLUTCH DR'VER 7CONTROL 1 7 I Q GATE I m LEFT 52 A OUTPUT sTEER L. T DRIVER CLUTCHNON-CONTROLLING I F T 7 CONTROL CLUTCH GATE COMPUTER s VENT HRS rfi'erzSanders,

- Byc/'zzj '6. fift /0111.6 xi flfleffi ifla nfe/a' {4M #4144 ATTORNF YPATENTED APR 1 7 I975 SHEET 2 BF 3 l I l u/mflm ATTORNEY PAIENI nAPRmm V3.727 710 sum 3 OF 3 AT TORNEY STEER CONTROL FOR A TRACK-LAYING VEHICLEThe invention herein described was made in the course of work under acontract or subcontract thereunder with the Department of Defense.

This invention relates to a steer control for a tracklaying vehicle andparticularly to a closed loop control having a track speed feedback. I

It has been common practice to employ cross-drive transmissions intrack-laying vehicles and to accomplish steering by driving by torquesensitive steer devices which include left and right steering clutchesand/or brakes in the transmission to effect a differential track speed.The torque sensitive steer devices have many inherent variables whichcan cause instability of the control which must be overcome by thevehicle operator. The variables include such things as clutch capacitywhich is variable with speed differential across the clutch pack, thetemperature and instant friction coefficient. Further, the return andexchange of track forces changes constantly with vehicle speed, type ofterrain and the radius of turn.

It is therefore a general object of this invention to provide a closedloop steer control for a track-laying vehicle. a

It is a further object of this invention to provide a steer control fora track-laying vehicle which employs a track speed feedback to thecontrol. I

It is another object to provide a steer control for a track-layingvehicle wherein the track speedv differential is a function of manualsteer input and the actual track speeds.

The invention is carried out by providing in a steer control for atrack-laying vehicle, variable capacity torque transmitting devices foreffecting a track speed differential, a steer selector, a track speedsensor and apparatus for regulating the torque capacityof the devices asa function of track speed and the desired turning rate to establish atrack speed differential. The invention further comprehends pressureresponsive devices for effecting track speed differential and anelectronic control circuit for computing the pressures to be applied tothe devices for establishing a desired track speed differential.

The above and other advantages will be made more apparent from thefollowing specification taken in conjunction with the accompanyingdrawings wherein like reference numerals refer to like parts andwherein:

FIG. 1 is a block diagram of a steer control circuit according to theinvention;

FIG. 2 is a schematic diagram of the steer lever transducer of FIG. 1;

FIG. 3 is a schematic diagram of the gate control and trigger circuit ofFIG. 1;

FIG. 4 is a schematic diagram typifying the gate circuits and outputdriver of FIG. 1;

FIG. 5 is a schematic diagram of the steer amplifier of FIG. 1;

FIG. 6 is a schematic diagram of the average speed computer of FIG. 1;

FIG. 7 is a schematic diagram of the fastest speed computer of FIG. 1;

FIG. 8 is a schematic diagram of the integrator of FIG. 1;

FIG. 9 is a schematic diagram of the summing circuit of FIG. I;

FIG. 10 is a schematic diagram of the 30 rpm switch of FIG. 1; and,

FIG. 11 is a schematic diagram of the non-controlling clutch computer ofFIG. 1.

The preferred embodiment of the control described herein is intended toapply to a cross-drive transmission of the type illustrated in theUnited States Patent to Christensen No. 2,930,257 which includes leftand right steering clutches which are selectively actuated to providesteering in one direction or the other. Each steering clutch is normallydisengaged for straight ahead driving and pressure is applied to theright steer clutch, for example, for a right turn and the pressure onthe clutch is proportioned according to the degree of steering or theturning rate desired. The controlling clutch is connected throughdifferential gearing to output shafts so as the right output shaftspeed, for example, is retarded, the left output shaft will be increasedin speed by a like amount to establish a track speed differential. Theinvention is not, however, limited to that particular arrangement, butrather is adaptable to other steering arrangements including variabletorque capacity torque transmitting devices, i.e., brakes or clutches,for effecting track speed differential.

The control depicted in FIG. I is intended to control steering clutchpressure in such a manner that a desired track speed differential isaccomplished and to that end, computes a pressure for the controllingclutch ac I cording to the following equation:

where K, 40 psi L= steer lever position (variable from 0 to l N, averagetrack speed N,= fastest track speed the K term is limited to i 20 psimaximum and the term is negative only. The non-controlling clutch issubjected to a pressure according to the following equation:

where P,, is limited to a minimum of 10 psi.

The equation for P is designed to provide, initially, upon a steerrequest, a pressure equal to the K,, L, and K, terms to approximate apressure slightly greater than that required to achieve the desiredsteering rate. The expression N,,L represents the requested turning ratewhile the expression N, N represents the actual turning rate or trackspeed differential. The K term then represents a pressure correspondingto the difference between the desired rate and the actual rate which maybe termed steer error." That term then tends to modulate the pressure toeliminate steer error. The K term represents the integral of steer errorand slowly but accurately provides a fine pressure control to eliminatethe steer error. The equation for P provides a minimum pressure on thenon-controlling clutch to keep the fluid motor of the non-controllingclutch filled during a turning maneuver. For example, during a rightturn the left steer clutch will be maintained filled and is thereforekept in readiness for rapid response should a left steer be required. Inthe event the vehicle enters a skid condition the term P becomesnegative. The right steer clutch of course cannot have a negativepressure but the pressure on the left steer clutch increases to reversethe sense of the track speed differential, as if for a left steer, sothat the skid condition is automatically corrected. This correction willoccur even if the steer position lever is held in the right steerposition.

Referring to FIG. 1 right and left track speed sensors and 12 detect therotational speed of the right and left output shafts respectively of thecross-drive transmission. These sensors are preferably of the wellknowntoothed wheel variable reluctance electromagnetic transducer type. Thesensor outputs are fed to frequency-to-voltage converters 14 and 16which produce on lines 18 and 20 DC analog signals proportional to rightand left track speeds respectively. Both speed signals are fed to anaverage speed computer 22 which produces on line 24 a DC signalproportional to the negative value of the average track speeds. Thespeed signals are also fed to a fastest speed computer 26 which produceson line 28 an analog signal proportional to the fastest track speed N, Asteer lever transducer 30 is controlled by a manually operated steerlever or steering wheel and produces on line 32 a DC signal having amagnitude and polarity depending upon the amount and direction of therequested steer respectively. That signal is fed to a steer amplifier 34which produces on output line 36 a positive signal L proportional to thedegree of steering or turning rate requested. The line 24 carrying thesignal N,, is connected as an input to the steer lever transducer whichmultiplies that signal according to the steer lever position to produceon line 38 a signal N,,L.

An integrator 40 has as inputs the signals on lines 24, 28 and 38 andproduces an output on line 42 proportional to A summing circuit 44includes as inputs the signals on lines 24, 28, 36, 38 and 42 and asignal proportional to K K where K is an offset voltage'for a purpose tobe described. These inputs are appropriately added or subtracted toproduce on line 46 an output signal proportional to P K A 30 rpm switch48 has lines 24 and 28 as inputs and produces on its output line 50 avoltage signal equivalent to 40 psi whenever N, N exceeds 30 rpm. Theline 50 is connected as an input to the summing circuit 44 to subtract40 psi from the pressure P whenever the 30 rpm switch is activated,thereby preventing an excessive turning rate.

A non-controlling clutch computer 52 has as its input the signal P K onthe line 46 and produces on line 54 an output signal P K A series ofgates includes gates 56 and 58 having the line 46 as inputs and gates 60and 62 having the line 54 as inputs. The outputs of the gates 56 and 60are connected to an output driver 64 which energizes a solenoid 66 inthe right steer clutch control. Similarly, the outputs of the gates 58and 62 are connected to an output driver 68 which energizes a solenoid70 in the left steer clutch control. The solenoids operate hydraulicvalves to provide a pressure according to solenoid. current as morefully described in the United States Patent to Schaeffer No. 3,225,619.The electrically modulated pressure is then applied to a steer clutch tocontrol the torque capacity thereof. A minimum solenoid current isrequired to initiate a clutch pressure.'The offset voltage K is providedto afford that minimum current. Any additional solenoid current as aresult of the signal P provides a clutch pressure proportional to P Agate control and trigger circuit 72 has its input connected to line 32.An output line 74 is connected to gates 56 and 62 and a second outputline 76 is connected to gates 60 and 58. Normally the signals on thelines 74 and 76 are such that the gates will be disabled and thesolenoids will be deenergized. When, however, the steer lever is movedfrom its neutral position to indicate a desired steer direction, one ofthe lines 74 or 76 will be energized to enable its respective gates. Forexample, when a right turn is indicated, the line 74 will be deenergizedto enable gates 56 and 62 so that the signal P K, will be passed to theoutput driver 64 and the right steer clutch will be controlled accordingto the pressure P Similarly, the signal corresponding to the pressure PK will pass gate 62 to effect a low pressure application to the leftsteer clutch except during a skid condition when the left clutchpressure becomes high. Conversely, when a left steer is selected theline 76 will be energized to energize the steer clutches in the oppositemanner. The gate control and trigger circuit 72 has a third output 78which is energized when steering is requested. Line 78 is connected tothe integrator 40 and serves to render the integrator inoperative untilsteering is requested, whereupon integration is allowed to occur.

FIG. 2 depicts the steer lever transducer 30. A potentiometer isconnected between l5v and +1 5v and has a grounded center tap. A slidetap 102 controlled by steer lever 104 is connected to the line 32 toprovide on line 32 a signal corresponding in polarity and magnitude tothe direction and degree of turning requested by the steer leverposition. A second potentiometer 106 has both ends connected to line 24carrying the signal N, and has a grounded center tap. The slide tap 108controlled by the steer lever 104 is connected to line 38 to provide thesignal N,,L which is the product of the average track speed and therelative lever position.

FIG. 3 depicts the gate control and trigger 72 which comprises an arrayof operational amplifiers 110, 112 and 114 each arranged as a triggercircuit. The amplifier 1 10 has its positive input connected through aninput resistor 116 to the line 32 and its negative input is connectedthrough resistors 118 and 120 to +1 5v and ground respectively. Theamplifier output is connected to line 76. The amplifier 112 has itspositive input connected through input resistors 122 and 124 to l 5v andground respectively and its negative input is connected through inputresistor 126 to line 32. The output is connected to line 74. For aneutral steer position, the line 32 will be at ground potential and bothamplifiers and 112 will have a negative output. The components aresoselected that a lever movement of less than 1 in either direction willactivate neither trigger circuit, thereby providing a neutral deadband.A larger lever movement will cause either amplifier 110 or 112,depending on the polarity of line 32, to trigger to impress a positivevoltage. on its output. Thus, for a right steer request, the line 74will become positive and for a left steer request, the line 76 willbecome positive to enable the corresponding gate circuits.

The amplifier 114 has its positive terminal connected to ground throughresistor 128 and the negative terminal is connected to -l5v throughresistor 130. The lines 74 and 76 are connected through diodes 132 and134 respectively and through an input resistor 136 to the negative inputterminal. Thus, when the lines 76 and 74 are each at a negative voltage,the amplifier output on line 78 will be positive. When, however, eitherof the lines 74 or 76 becomes positive, the amplifier 114 will switch toa negative output to trigger the integrator 40.

FIG. 4 depicts a gate circuit and output amplifier and a solenoid whichare representative of all the gate circuits and both output drivers. Thegate circuit 56 comprises a field effect transistor (FET) 138 having itssource and drain electrodes connected to lines 46 and 140 respectivelyand its gate electrode connected through a diode 142 to line 74. The.gate electrode is also connected through a resistor 144 to the line 46.The F ET will be conductive except when a large negative voltage isapplied to its base. Since the line 74 is normally negative, the F ETwill normally be turned off. When, however, the line 74 becomespositive, the negative gate potential is removed and the FET conducts sothat the signal P K on line 46 is passed to the line 140.

The output driver 64 includes an operational amplifier 146 connected asan inverting amplifier and has its positive terminal connected throughresistors 148 and 150 to +l 2v and ground respectively while itsnegative input terminal is connected through input resistors 152 and 154to the line 140 and to the line 156 which is the input from the gate 60.The negative input terminal is also connected to a feedback resistor158. The output of the amplifier 146 then will be 12v P K assuming thatthe gate 56 is conducting. a

A power amplifier comprises a transistor 160 having its collectorconnected to +l2v and its emitter connected through resistor 162 toground. The emitter is also connected to the base of a power transistor164 having its emitter connected through a power resistor 166 to +1 2vand the collector is connected to the sole- .noid 66. A spikesuppressing diode 168 is connected between ground and the collector ofthe transistor 164. When 12v is applied to the base of the transistor160, it will conduct to maintain the power transistor 164 turned off.When, however the l2v input signal is reduced by the P K signal, thetransistor 160 will become less conductive and the transistor 164 willconduct according to the value of P,. K to energize the solenoid 66accordingly. v

FIG. shows the steer amplifier 34 which includes a pair of operationalamplifiers 170 and 172.conriected as inverting amplifiers. The amplifier170 has its positive input terminal grounded through a resistor 174 andits negative terminal connected to line 32 through input resistor 176.The negative terminal is connected through a feedback resistor 178 tothe output line 36 and through a diode 180 to the amplifier output. Theamplifier 172 has its negative terminal connected through a feedbackresistor 182 to line 36 and through a diode 184 to the amplifier output.The positive input terminal is connected through resistors 186 and 188to ground and line 32 respectively. When the line 32 goes positive, theamplifier produces a proportionate negative output which is appliedthrough the diode to line 36. When line 32 goes negative, the amplifier172 produces a negative output which is applied to line 36 through thediode 184. The signal on line 36 accordingly, corresponds to -L. I

FIG. 6 shows the average speed computer 22 which includes an operationalamplifier 190 connected as an inverting amplifier and has its positiveinput terminal connected through a resistor 192 to ground and itsnegative input terminal connected through resistors 194 and 196 to lines20 and 18 respectively which carry positive signals corresponding toleft and right track speeds. The negative input of the amplifier is connected through a feedback resistor 198 to the amplifier output. Afiltering capacitor 200 is placed in parallel with the resistor 198 toeliminate ripple. The resistors are chosen to provide an amplifier gainof 56 so that the output signal on line 24 will be N. which is thenegative average of the track speeds.

The fastest speed computer 26 is shown in FIG. 7 and comprises a pair ofoperational amplifiers 202 and 204 connected as non-invertingamplifiers. The positive input of the amplifier 202 is connected throughre sistors 206 and 208 to ground and line 20 respectively. The negativeinput is connected to ground through a resistor 210 and is connectedthrough a resistor 212 and a diode 214 to the amplifier output. Acapacitor 216 is in parallel with the feedback resistor 212 to eliminateripple. The junction of the diode 214 and the resistor 212 is connectedthrough an FET 215 to an output line 28 which is connected to groundthrough a resistor 218. The gate electrode of the FET 215 is connectedthrough a resistor 217 to the cathode of the diode 214 and is furtherconnected through a diode 219 to the line 74. The FET operates the sameas the FET in the gate circuit of FIG. 4 and is arranged to conduct onlywhen the line 74 is positive, i.e., when a right steer is requested, totransmit the left track speed signal to line 28. The amplifier 204 isarranged in exactly the same manner except that its input is connectedto the line 18, and the gate of the corresponding F ET is connected tothe line 76. The output line 28 therefore will carry a voltagecorresponding to the left or right track speed, whichever shouldnormally be higher for the requested steer direction, and is termed thesignal N FIG. 8 depicts the integrator 40 which includes an operationalamplifier 220 having its positive input connected to ground throughresistor 221 and its negative input connected through resistors 222, 224and 226 to the lines 24, 38 and 28 respectively. The negative input isalso connected through an integrating capacitor 228 and a diode 230 tothe amplifier output. The output line 42 is connected at the junction ofthe diode and capacitor and is connected to ground through a resistor232. An FET 234 is connected across the capacitor 228 and has its gateconnected through a resistor 236 to the amplifier output. The gate isalso connected through a diode 238 to line 78 which carries a triggervoltage from the gate control and trigger circuit 72. During a neutralposition of the steer lever, the line 78 is positive so that the FETgate is conducting and the integrator output is held at zero potential.When the steer lever is moved to a steer condition, the line 78 goesnegative to turn off the FET so that the integration will begin. Sincethe integrator inputs are N,,, N L and +N the integrator output will bethe K factor being derived from the integrator gain. Due to the polarityof the diode 230, only a negative integrator output is permitted.

A summing circuit 44 as shown in FIG. 9 includes an operationalamplifier 240 having its positive input connected through resistors 242and 244 to ground and line 42 respectively and its negative inputconnected through input resistor 246 to a potentiometer 248 which isconnected between 15v and ground to provide a signal equivalent to -(K,K The negative input is also connected through resistors 250, 252, 254,256 and 258 which are connected to lines 38, 24, 50, 28 and 36respectively. The negative terminal is connected through a firstfeedback resistor 260 and a diode 262 to the amplifier output and thejunction point of the diode and the feedback resistor is connected tothe output line 46, and is also connected through a resistor 264 toground. The negative input is further connected to a second feedbackresistor 266 and a diode 268 to the amplifier output and the junction ofthe diode 268 and the feedback resistor 266 is connected through aresistor 270 to ground. Since the positive input of the amplifier has asits input the K term from the integrator and the negative input terminalhas as input signals K K N L, N N and -L, the output signal on line 46will be the signal corresponding to the pressure P. K Further, when asignal is present on line 50 from the 30 rpm switch, the output signalwill be accordingly reduced.

The 30 rpm switch 48 is shown in FIG; 10 and comprises an operationalamplifier 272 connected as a trigger circuit having its negative inputconnected to a voltage divider 274 which is connected between +v andground. The positive input is connected through resistors 276 and 278 tolines 24 and 28 which carry the signals N and N; respectively. Theoutput of the amplifier is connected through a diode 280 and apotentiometer 282 to ground, the slide tap of the potentiometer beingconnected to the output line 50. Normally, the amplifier output isnegative and the diode 280 is backbiased so that the line 50 is atground potential. When, however, signal N, exceeds N by an amountequivalent to 30 rpm, the circuit will trigger to provide a positiveoutput which is applied to line 50.

The non-controlling clutch computer 52 is shown in FIG. 11 and includesa pair of operational amplifiers 284 and 286 connected as unity gaininverting amplifiers. The amplifier 284 has its positive input groundedthrough resistor 288 and its negative input is connected throughresistors 290 and 292 to line 46 carrying the signal P K and toapotentiometer 294 connected between l5v and ground. The negativeterminal is also connected through a feedback resistor 296 and a diode298 to the amplifier output. The output 6 is zero (P K the amplifieroutput is sufficient to cause fully apply clutch pressure, and when thevoltage on line 46 is K (P O) the output line will have a signalcorresponding to K 10 psi. Thus during askid condition when the signal Pbecomes negative, the outside steer clutch will be energized to correctthe skid. The amplifier 286 is connected in the same manner as theamplifier 284 except that the negative input is connected through aresistor 300 to a potentiometer 302 providing a signal equivalent to 10psi K The output line 54 will then carry a signal corresponding to 10psi K or 10 psi P K whichever is larger.

It will thus be seen that the control circuit described herein providesa means for sensing track speeds and steer lever position so that for agiven lever position, the appropriate steer clutch will be supplied witha pressure such that the actual track speed differential will correspondto the turning rate requested by the steer lever position, and thatduring a skid condition the opposite clutch will be pressurized tocorrect the skid.

The embodiment of the invention described herein is for purposes ofillustration and the scope of the invention is intended to be limitedonly by the following claims:

It is claimed: 1. A closed loop steer control for a track laying vehiclefor controlling track speed differential as a function of relative trackspeeds comprising a pair of variable torque capacity torque transmittingdevices each actuable for effecting steering by positively controllingtrack speed differential,

steer selector means for selecting the desired degree and direction ofsteering,

means for sensing the track speeds, and

means responsive to the sensing means and the steer selecting means forsimultaneously regulating the torque capacity of the devices as afunction of the track speeds for establishing a track speed differentialconsistent with the desired degree and direction of steering.

2. A closed loop steer control for a track laying vehicle forcontrolling track speed differential as a function of relative trackspeeds comprising a pair of pressure responsive torque transmittingdevices having torque capacities varying with pres sure, andsimultaneously actuable for effecting steering by positively controllingtrack speed differential,

manually operated steer selecting means for selecting the desired degreeand direction of steering, means for sensing the track speeds, and

pressure controlling means responsive to the sensing means and the steerselecting means for simultaneously regulating the pressures on thedevices as a function of the track speeds for establishing a track speeddifferential consistent with the desired degree and direction ofsteering.

3. A closed loop steer control for a track laying vehicle forcontrolling track speed differential as a function of relative trackspeeds comprising a pair of variable torque capacity torque transmittingdevices each actuable for effecting steering by positively controllingtrack speed differential,

means associated with each device responsive to an its respective devicefor primarily regulating the electrical control signal for varying thetorque pressure on said respective device as a function of capacity ofeach device, the track speeds for establishing a track speed difmanuallycontrolled electrical selector means for ferential consistent with thedesired degree and selecting the desired degree and direction ofsteerdirection of steering, and including means for aping, plying asignal to the other of the pressure responelectrical means for sensingthe track speeds, and sive means for regulating the other of the devicesan electronic controller responsive to the sensing at a low pressure tohold it in readiness for actuameans and the steer selecting means forproducing tion for a reversal of track speed differential. theelectrical control signal and applying the signal 5. A closed loop steercontrol for a track laying vehito the signal responsive means to actuatetheir cle for controlling track speed differential as a functionrespective devices and for simultaneously regulatof relative trackSpeeds p ng ing the torque capacity of the devices as a function a P ofSelectively engageabie Variable torque of the track speeds forestablishing a track speed p y 'q transmitting devices, differentialconsistent with the desired degree and i5 Pressure responsive means forSimultaneously di i f Steering lating the devices for effecting steeringin one 4. A closed loop steer control for a track laying vehidirection,cle for controlling track speed differential as a function electronicconii'oi means regiiiaiing the Pressure f l i track Speeds comprisingresponsive means for establishing a track speed a pair f variable torquecapacity torque transmitting differential according to a desired vehicleturning devices each actuable for effecting a track speed t the Controlmeans including manuaiiy diff i l trolled electrical selector means forselecting the pressure responsive means associated with each directionof steering and i desired turning fate, device regulated by anelectrical control signal for electrical means f" n ng the "j Speeds,varying thetorque capacity of each device means effective during aninitial steering phase for manually controlled electrical selector meansfor effeifiing Pressures on i gn devices for selecting the desireddegree and direction of steer- Causing track Speed differential n meansf ing, modulating the pressure on each device according electrical meansfor Sensing the track speeds and to the difference between the desiredturning rate an electronic controller responsive to the sensing and theactual lummg rate regulate i track means and the steer selecting meansfor producing spe ed dlfferfmt-ial to that required to achieve the theelectrical control signal and applying the signal desired turning ratetoone of the pressure responsive means to actuate

1. A closed loop steer control for a track laying vehicle forcontrolling track speed differential as a function of relative trackspeeds comprising a pair of variable torque capacity torque transmittingdevices each actuable for effecting steering by positively controllingtrack speed differential, steer selector means for selecting the desireddegree and direction of steering, means for sensing the track speeds,and means responsive to the sensing means and the steer selecting meansfor simultaneously regulating the torque capacity of the devices as afunction of the track speeds for establishing a track speed differentialconsistent with the desired degree and direction of steering.
 2. Aclosed loop steer control for a track laying vehicle for controllingtrack speed differential as a function of relative track speedscomprising a pair of pressure responsive torque transmitting deviceshaving torque capacities varying with pressure, and simultaneouslyactuable for effecting steering by positively controlling track speeddifferential, manually Operated steer selecting means for selecting thedesired degree and direction of steering, means for sensing the trackspeeds, and pressure controlling means responsive to the sensing meansand the steer selecting means for simultaneously regulating thepressures on the devices as a function of the track speeds forestablishing a track speed differential consistent with the desireddegree and direction of steering.
 3. A closed loop steer control for atrack laying vehicle for controlling track speed differential as afunction of relative track speeds comprising a pair of variable torquecapacity torque transmitting devices each actuable for effectingsteering by positively controlling track speed differential, meansassociated with each device responsive to an electrical control signalfor varying the torque capacity of each device, manually controlledelectrical selector means for selecting the desired degree and directionof steering, electrical means for sensing the track speeds, and anelectronic controller responsive to the sensing means and the steerselecting means for producing the electrical control signal and applyingthe signal to the signal responsive means to actuate their respectivedevices and for simultaneously regulating the torque capacity of thedevices as a function of the track speeds for establishing a track speeddifferential consistent with the desired degree and direction ofsteering.
 4. A closed loop steer control for a track laying vehicle forcontrolling track speed differential as a function of relative trackspeeds comprising a pair of variable torque capacity torque transmittingdevices each actuable for effecting a track speed differential, pressureresponsive means associated with each device regulated by an electricalcontrol signal for varying the torque capacity of each device, manuallycontrolled electrical selector means for selecting the desired degreeand direction of steering, electrical means for sensing the trackspeeds, and an electronic controller responsive to the sensing means andthe steer selecting means for producing the electrical control signaland applying the signal to one of the pressure responsive means toactuate its respective device for primarily regulating the pressure onsaid respective device as a function of the track speeds forestablishing a track speed differential consistent with the desireddegree and direction of steering, and including means for applying asignal to the other of the pressure responsive means for regulating theother of the devices at a low pressure to hold it in readiness foractuation for a reversal of track speed differential.
 5. A closed loopsteer control for a track laying vehicle for controlling track speeddifferential as a function of relative track speeds comprising a pair ofselectively engageable variable torque capacity torque transmittingdevices, pressure responsive means for simultaneously regulating thedevices for effecting steering in one direction, electronic controlmeans regulating the pressure responsive means for establishing a trackspeed differential according to a desired vehicle turning rate, thecontrol means including manually controlled electrical selector meansfor selecting the direction of steering and the desired turning rate,electrical means for sensing the track speeds, means effective during aninitial steering phase for effecting pressures on the regulated devicesfor causing a track speed differential, and means for modulating thepressure on each device according to the difference between the desiredturning rate and the actual turning rate to regulate the track speeddifferential to that required to achieve the desired turning rate.